1. Fields of the Invention
The present invention relates to an electrical cable. It is typically, but not exclusively, employed as a high-voltage electrical transmission cable or overhead cable, commonly called an overhead line (OHL), to transmit power.
More particularly, the invention relates to an electrical cable that reduces the incidence of the corona effect.
2. Description of the Related Art
Overhead lines are traditionally formed by bare conductive elements held by an appropriate array of pylons. These lines are conventionally intended to transmit high-voltage (225 to 800 kV) AC electrical power. Each conductor therefore has a diameter of a few centimeters and may be composed of a metal layer or, more frequently, of several assembled metal wires. Along the bare conductor an effect, called the corona effect, is always observed. Specifically, any conductor or line subjected to a high voltage will exhibit the corona effect. Once the electric field at the surface of the conductor, especially depending on local radii of curvature, becomes sufficiently high locally (i.e. higher than the dielectric strength of moist air, about 10 kV/cm; or even higher than the dielectric strength of dry air, about 30 kV/cm), the air ionizes and a luminous corona forms around the conductor.
One of the consequences of the corona effect is the production of noise. However, when the conductor is dry a very limited amount of noise is generated, which is practically negligible in terms of the discomfort it causes. In contrast, when the conductor is wet the noise generated is much louder and it is a source of notable discomfort and a great deal of annoyance for people passing by or living next to this type of conductor. Specifically, under these conditions, the conductivity of the air increases and thus the ionization is more intense and more efficient.
The corona effect also results in power losses and the electromagnetic radiation, acoustic noise, and power losses generated may have health effects.
In order to overcome this problem, one solution consists in insulating the conductors by covering them with a layer of an insulating plastic material, so as to prevent electrical contact between the biased metal parts and the air, thereby suppressing the corona effect.
In another approach, it has been suggested to increase the diameter of the cable. Document DE 4 424 007 describes an electrical cable especially comprising at least one layer of a conductive element intended to conduct the power transmitted by the cable in its operational configuration, and a supporting element formed from carbon fibers. Said supporting element encircles said layer of the conductive element or is positioned between two layers of said conductive element, which layers are intended to conduct the power transmitted by the cable in its operational configuration. Increasing the diameter of the cable reduces the tangential electric field and therefore the incidence of the corona effect. However, this type of cable has the drawback of either being too heavy or not adequately reducing the incidence of the corona effect. Furthermore, its current capacity is limited.